Petroleum refining and petrochemical processes frequently involve the processing of fluids over particulate solids contained within a pressure vessel. Internal partitions can subdivide the interior of a pressure vessel into different chambers to permit staged or multiple contacting operations within a single vessel. These partitions routinely take the form of, or are used in conjunction with, collection or distribution grids. Process requirements, such as the collection and distribution of fluids, regularly dictate the employment of flat partitions. Concomitantly, pressure vessels usually are closed by rounded “heads” at each end. The rounded head and flat interior partitions at each end of a vessel create a head space whose configuration is not suited to process purposes, risking contamination or deterioration of the process if this head space itself becomes contaminated and subsequently mixes with the high purity process fluid.
Further, flat partitions are subject to structural damage from differential pressures of as little as 15 kPa or even less across the partition. Structural damage to a partition has the potential to create leaks across the partition or in associated distribution/collection piping.
Thus, maintaining structural integrity of interior end partitions requires pressure balancing between the head space and the adjoining volume on the process side of the partition. The head space can serve as an equalization chamber through a small opening or port in the partition communicating head fluid to and from the process chamber on the opposite side of the partition. However, this arrangement risks some inefficiency in the process through process fluid passing into the head space, resulting in some loss in yield. Alternatively, subsequent reversal of flow of the head fluid into the process chamber has the potential to contaminate the process and final product.
A specific technology which illustrates the above problem is the simulated moving bed (“SMB”) adsorbent process described in U.S. Pat. No. 2,985,589. The process distributes and collects process streams from multiple chambers with multiple zones, or beds, of adsorbent defined by internal partitions located within a pressure vessel and arranged as distribution/collection grids. Periodic shifting of the input and effluent streams through the chambers simulates movement of the adsorbent and permits delivery or withdrawal of process streams with a desired concentration via flat distribution grids.
The head space resulting from the flat distribution grids and a concave end is flushed by a small flow of a flush fluid, usually comprising a desorbent material. A desorbent material normally is selected so that passage of this material into the adsorbent bed through a grid opening does not contaminate the products of the process.
However, the periodic shifting of the input and effluent streams through the chambers of adsorbent can effect a buildup of contaminants in the desorbent through leakage through the grid opening, particularly in the bottom head of the chamber.
Further, the addition of desorbent to the adsorbent bed through the grid opening can interfere with the optimization of purity and recovery by taking up adsorbent capacity and hindering an accurate accounting of flow through the adsorbent beds.
U.S. Pat. No. 5,595,665, incorporated herein by reference in its entirety, addresses some of these issues by channeling the fluid generated by a head flush into a low volume chamber (referred to herein as “snorkel”) in the head space and withdrawing fluid from the pressure vessel through the snorkel. Withdrawing fluid generated by the head flush and channeling the fluid through the snorkel reduces or eliminates the circulation of fluid between the equalization chamber and the adjacent process chamber and minimizes the amount of contamination that can result from any circulation of fluid resulting from pressure fluctuations. The withdrawal of fluid through the snorkel also provides a non-contaminating path for withdrawing leakage from the equalization chamber of the vessel.
However, this feature fails to address the fluid that is used as the flush fluid in the system. More specifically, the use of desorbent as a flush fluid unnecessarily increases the energy requirements of the process as the desorbent withdrawn fluid must be recovered and purified via fractionation before re-use in the process.
Therefore, there remains a need for an effective and efficient process for flushing of the various components of a simulated moving bed process.